_ch_10d+lecture_presentation

Chapter Overview

• The Nervous System: Sensory Systems

  • Focus on the ear and its functions related to hearing and equilibrium.

Chapter Outline

10.4 The Ear and Hearing

10.5 The Ear and Equilibrium

Learning Outcomes

• Understand the two sensory systems of the ear.

10.4 The Ear and Hearing

Anatomy of the Ear

• Outer Ear

  • Pinna

  • External auditory meatus

• Middle Ear

  • Tympanic membrane

  • Ossicles: malleus, incus, stapes

  • Oval window

  • Round window

• Inner Ear

  • Cochlea for sound transduction

  • Vestibular apparatus

  • Eustachian tube for pressure equalization (auditory tube)

The Nature of Sound Waves

• Definition

  • Mechanical waves caused by air molecule motion

• Characteristics

  • Wavelength: distance between compressed regions

  • Amplitude: loudness, measured in decibels (dB) on a logarithmic scale

  • Frequency: pitch of sound, measured in Hertz (Hz), with a typical range of 20 – 20,000 Hz; optimal hearing is 1000 - 4000 Hz

Sound Amplification in the Middle Ear

• Mechanism

  • Tympanic membrane vibrates at sound frequency

  • Movement of ossicles increases vibration

  • Movement of oval window amplifies sound from a larger surface area (tympanic membrane) to a smaller one (oval window)

Signal Transduction for Sound

• Process

  • Conversion of sound energy into action potentials begins in cochlea

• Anatomy of Cochlea

  • Flows of fluid: perilymph (scala vestibuli and scala tympani), endolymph (scala media); high potassium concentration in endolymph

  • Hair Cells

  • Inner hair cells act as receptors

  • Outer hair cells regulate sensitivity

  • Stereocilia bent to create signals for sound information

Neural Pathways for Sound

• Connection

  • Hair cells synapse with afferent neurons of cranial nerve VIII (cochlear nerve)

  • Signals travel to brainstem, thalamus, and auditory cortex; organized tonotopically

Clinical Defects: Deafness

• Types

  • Conductive Deafness: problems in external/middle ear preventing sound wave conduction

  • Sensorineural Deafness: issues with transduction in inner ear or cranial nerve VIII

  • Central Deafness: damage along neural pathways to CNS

10.5 The Ear and Equilibrium

Anatomy of the Vestibular Apparatus

• Contains semicircular canals for rotational balance and utricle/saccule for linear acceleration detection

The Semicircular Canals and Rotational Transduction

• Three canals

  • Anterior: movement up/down

  • Posterior: left/right movement

  • Lateral: side-to-side turning

• Detection Mechanism

  • Hair cells in ampullae (cristae); response to fluid motion caused by head movement

Detection of Movement

• Resting or constant motion produces a tone in hair cells

• Acceleration

  • Head rotation affects hair cells through mechanical bending

  • Hyperpolarization occurs when stereocilia bend toward short; depolarization when they bend toward tall

The Utricle and Saccule and Linear Acceleration

• Function

  • Utricle for forward/backward movement

  • Saccule for up/down detection

• Mechanism

  • Movement causes hair cells to bend, resulting in neural signaling through similar ion channel interactions (K+ and Ca2+) as in the cochlea

Conclusion

• Integration of auditory and vestibular functions ensures effective response to sound and balance. Understanding the anatomy and mechanisms is crucial for diagnosing and treating auditory and balance disorders.